The present invention relates to a method of shift control in an automatic transmission to automatically shift the speed range corresponding to changes in engine power and vehicle speed.
In an automatic transmission, the speed range is automatically shifted corresponding to changes in traveling state to attain desirable traveling characteristics. A point representing a traveling state is shown on a shift map point representing a traveling state is shown on a shift map having shift-up lines and shift-down lines which are established based on vehicle speeds and engine powers (or throttle openings, intake vacuum pressures etc.). The shift control is conducted based on the movement of the point representing the traveling state on the shift map. Such a control method as described above is disclosed in the Japanese Patent Laid-Open Publication No. 61(1986)-189354.
The shift map is established considering many road conditions such as open roads, highways and mountainous roads. Because of so many conditions to be considered, the establishment of the map is complicated.
The shift lines on the shift map are established so as to cope with the many conditions. In order to fully utilize the engine power, the speed range is shifted up when a point representing the traveling state moves toward higher speed direction crossing the shift line, and it is shifted down when the point moves toward lower speed direction crossing the shift line. If the up-shift and down-shift are determined based on one common shift line, the up-shift and the down-shift can happen frequently to deteriorate the driving feeling when the point moves in the vicinity of the shift line. Accordingly, the shift-up line is usually established on higher speed side than the shift-down line. A hysteresis zone established between the lines can moderate the frequency of the shifts.
It is desirable to include a hysteresis zone to reduce the frequency of the shifts. However, if the zone is too broad, the following problem may arise. For example, when the speed range is shifted up during running on an ascending slope, the downshift may not be conducted until the vehicle speed is greatly decreased. Accordingly, the driving force becomes poor to deteriorate the driveability. Further, the torque converter is operated in a range of low speed ratio (speed ratio=output speed/input speed) to create large slip and heat in the torque converter.